dmaengine: Update documentation for inline wrapper

Commit 16052827d98fbc13c31ebad560af4bd53e2b4dd5 ("dmaengine/dma_slave:
introduce inline wrappers") introduced some wrappers, but there is still
a reference to the old function.

Update the documentation to use the wrapper, and add a missing "()" to a
function name.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>